1. Field of the Invention
The present invention relates to a semiconductor device and a semiconductor device using the same and having a photoelectric conversion function and, more particularly, to a semiconductor device wherein a plurality of switch means for transferring signals upon switching, and a plurality of matrix wirings respectively connected to the plurality of switch means are formed on a single substrate. Note that the semiconductor device having the photoelectric conversion function of the present invention is suitably used in an image reading apparatus serving as an input section of, e.g., a facsimile system, an image reader, a digital copying machine, an electronic chalkboard, and the like.
2. Related Background Art
In recent years, in order to realize a compact, high-performance image reading apparatus such as a facsimile system, an image reader, or the like, an elongated line sensor having an equal-magnification optical system used as a photoelectric conversion apparatus has been developed. Conventionally, in a line sensor of this type, a plurality of photoelectric conversion elements are aligned in an array, and signal processing ICs (Integrated Circuits) comprising switching elements, are connected to the photoelectric conversion elements. However, 1728 photoelectric conversion elements are required for an A4 size to comply with the facsimile G3 standards, and, hence, a large number of signal processing ICs are also required. For this reason, the number of mounting steps is also increased, resulting in high manufacturing cost and poor reliability. Thus, matrix wirings are adopted to decrease the number of signal processing ICs and to decrease the number of mounting steps.
FIG. 1 is a view for explaining a structure of a photoelectric conversion apparatus having a matrix wiring structure.
The photoelectric conversion apparatus of FIG. 1 includes a photoelectric conversion element unit 101, a scanning unit 102, a signal processing unit 103, and a matrix wiring unit 104.
FIG. 2A is a plan view of a conventional matrix wiring unit, and FIGS. 2B and 2C are sectional views taken along lines A-A' and B-B' in FIG. 2A, respectively.
The matrix wiring unit shown in FIGS. 2B and 2C includes a substrate 201, individual electrodes 202 to 205, an insulating layer 206, common lines 207 to 209, and a through hole 210 for electrically connecting each individual electrode and the common line.
In this manner, in the photoelectric conversion apparatus having an m.times.n matrix wiring structure, the number of signal processing circuits in the signal processing unit 103 can be decreased to be equal to the number (n) of output lines of the matrix. Therefore, the signal processing unit can be made compact, and the cost of the photoelectric conversion apparatus can be decreased.
On the other hand, in a photoelectric conversion apparatus using a thin film semiconductor, photoelectric conversion elements and thin film transistors (to be abbreviated to as TFTs hereinafter,) serving as transfer circuits are formed on a single substrate in a single process to realize a compact, low-cost photoelectric conversion apparatus. In order to realize a further compact, low-cost apparatus, a so-called lensless type photoelectric conversion apparatus, in which a sensor directly detects light reflected by an original through a transparent spacer such as glass without using an equal-magnification fiber lens array, is also proposed.
However, a conventional photoelectric conversion apparatus having a matrix wiring structure described above involves the following technical subjects to be solved.
Since a very weak output of each photoelectric conversion element is read out through matrix wirings, crosstalk occurs among output signals unless a stray capacitance formed at intersections between output individual electrodes and common lines in the matrix is sufficiently decreased. This drawback imposes strict limitations on the selection of a material as an insulating interlayer, and dimensional designs of the matrix wiring structure.
Since the matrix common lines are formed to extend along an extending direction of the apparatus, a line sensor having a width corresponding to, e.g., an A4 size has a length of 210 mm. For this reason, crosstalk also occurs among output signals unless interline capacitance between the common lines is also sufficiently decreased. If this drawback is to be simply prevented, the matrix wiring unit undesirably becomes large in size.
Furthermore, a pitch between two adjacent output individual electrodes of the photoelectric conversion elements is, e.g., as narrow as 125 .mu.m in a photoelectric conversion apparatus having a resolution of 8 lines/mm. For this reason, crosstalk also occurs among output signals unless an interline capacitance between the individual electrodes is also sufficiently decreased.
In order to realize a photoelectric conversion apparatus, which can eliminate the above-mentioned drawbacks, is free from crosstalk among output signals, and comprises a compact matrix wiring structure, a photoelectric conversion apparatus shown in FIG. 3 is proposed. This apparatus is disclosed in, e.g., Japanese Patent Laid-Open No. 62-87864, European Patent Gazette 0256850, and the like.
FIG. 3 is a cross-sectional view showing a section of a photoelectric conversion apparatus having a matrix wiring unit according to the above-mentioned proposal.
In this case, photoelectric conversion elements, TFTs, and matrix wirings are formed on a single substrate in the same process using a thin film semiconductor.
The structure shown in FIG. 3 includes a photoelectric conversion element unit 1, an accumulation capacitor unit 2, a TFT unit 3, a portion 4 having an illumination window (shown in cross-section), a matrix wiring unit 5, a transparent spacer 6, an original 7, and a substrate 8. The photoelectric conversion element unit 1, the accumulation capacitor unit 2, the TFT unit 3, and the matrix wiring unit 5 indicate regions which are respectively occupied by a photoelectric conversion element, an accumulation capacitor, a TFT, and a matrix wiring formed on the substrate. Incident light indicated by an arrow 9 propagates through the illumination window (shown in cross-section), and irradiates the photoelectric conversion element unit 1 as reflected light 10.
Information light incident on the photoelectric conversion element unit 1 is converted to a photocurrent, and is accumulated in the accumulation capacitor unit 2 as electrical charges. After the lapse of a predetermined period of time, charges accumulated in the accumulation capacitor unit 2 are transferred by the TFT unit 3 toward the matrix wiring unit 5.
On the substrate 8, a first conductive layer 12 of Al, Cr, or the like, a first insulating layer 13 of SiN, SiO.sub.2, or the like, amorphous silicon hydride (to be abbreviated to as an a-Si:H hereinafter) layer 14, an n.sup.+ -type a-Si:H doping layer 15, a second conductive layer 16 of Al, Cr, or the like, a second insulating layer 17 of a polyimide film or an SiN or SiO.sub.2 film, and a third conductive layer 18 of Al, Cr, or the like are sequentially formed.
The matrix wiring unit 5 includes individual signal wirings 19 and common signal wirings 18. A conductive layer 20, which can keep a constant potential, is formed on each intersection between the individual and common signal wirings 19 and 18 to be vertically sandwiched therebetween through the insulating layers 13 and 17.
In order to form the photoelectric conversion apparatus, the first conductive layer 12 of Al, Cr, or the like is deposited on the transparent substrate 8 of, e.g., glass by sputtering or deposition, and is patterned into a desired shape. The first insulating layer 13 of silicon nitride (SiN), the a-Si:H layer 14, and the n.sup.+ -type a-Si:H doping layer 15 are formed on the resultant structure by a known technique, e.g., plasma CVD. Thereafter, these three layers 13, 14, and 15 are patterned into a desired shape. Furthermore, the second conductive layer 16 of Al, Cr, or the like is formed by sputtering, deposition, or the like, and is patterned into a desired shape. In this case, the n.sup.+ -type a-Si:H doping layer 15 formed in the gap portion of the photoelectric conversion element and the channel portion of the TFT is removed by etching. Thereafter, the second insulating layer 17 of a polyimide film or an SiN film is formed on the second conductive film 16, and a contact hole is then formed. The resultant structure is patterned into a desired shape, as needed. Finally, the third conductive layer 18 of Al, Cr, or the like is formed on the second insulating layer 17 by sputtering, deposition, or the like, and is patterned into a desired shape.
In the photoelectric conversion apparatus manufactured in the above steps, the conductive layer 20, which can keep a constant potential is formed at each intersection between the individual and common signal wirings 19 and 18 excluding the through hole portion, so that formation of a stray capacitance between the individual and command signal wirings is prevented. In addition, although not shown, a wiring layer, which can keep a constant potential is formed between the individual and common signal wirings, so that formation of interline capacitances between the individual signal wirings and between the common signal wirings can be prevented. Therefore, the lines can be prevented from being capacitively coupled, and crosstalk among output signals can be avoided.
However, even in a photoelectric conversion apparatus with the above structure in which a conductive layer which can keep a constant potential is formed at each intersection between individual and common signal wirings, the following technical subjects are still left unsolved.
In this structure, a stray capacitance between the individual and common signal wirings can be suppressed. However, a new stray capacitance is formed between the conductive layer, for keeping a constant potential, and each individual signal wiring, and between the conductive layer and each common signal wiring.
Since the conductive layer for keeping a constant potential is formed on the entire surface of the matrix signal wiring unit excluding the through hole portions, the stray capacitance is formed in all the portions between the individual signal wirings and the conductive layer and between the common signal wirings and the conductive layer, and is not negligible in a practical application to realize a high-performance apparatus.
This problem will be described below with reference to FIG. 4. FIG. 4 shows an equivalent circuit of an accumulation type photoelectric conversion apparatus using a matrix wiring structure having a conductive layer for keeping a constant potential.
For example, when the matrix signal wirings are used at the output side of the accumulation type photoelectric conversion apparatus shown in FIG. 4, a stray capacitance 404 which is not negligible as compared to the value of a load capacitor 405 is generated, and transfer efficiency may be decreased.
In contrast to this, an S/N ratio, a dynamic range, and the like may be improved by increasing the capacitance of a accumulation capacitor 403. However, dimensions of the substrate of the photoelectric conversion apparatus are increased by an increase in accumulation capacitance, and the number of substrates per manufacturing batch is decreased. Note that reference numeral 401 designates a signal source, and 402 designates a switching means.
FIG. 5 and FIGS. 6A, 6B, and 6C are schematic cross-sectional views of European Patent Gazette 0296603 proposed by the present applicant.
An accumulation capacitance and a wiring unit are formed on the same portion on a substrate to decrease the width of the substrate of the photoelectric conversion apparatus.
A pattern arrangement of a photoelectric conversion apparatus will be described below with reference to FIG. 5.
In FIG. 5, a wiring pattern of a first layer, as a lowermost layer, is indicated by broken lines, a wiring pattern of a second layer is indicated solid lines, and a wiring pattern of a third layer, as an uppermost layer, is indicated by hatching.
The pattern shown in FIG. 5 includes a signal line matrix wiring unit 613, sensors 614, accumulation capacitances 616 formed in the signal line matrix wiring unit 613 and a gate wiring unit 619, transfer TFTs 617, reset TFTs 618, and illumination windows 620.
FIG. 6A is a cross-sectional view taken along a line A-A' in FIG. 5, FIG. 6B is a cross-sectional view taken along a line B-B' in FIG. 5, and FIG. 6C is a cross-sectional view taken along a line C-C' in FIG. 5.
The structure shown in FIGS. 6A to 6C includes a glass substrate 301, an insulating layer 303, an a-Si:H layer 304, an n.sup.+ -type a-Si:H doping layer 305, a first electrode layer 307, a second electrode layer 306 for forming gate electrodes, sensor gate electrodes of the accumulation capacitors, and the like. The structure also includes a second insulating layer 308, a third electrode layer 309 for deriving a wiring in a longitudinal direction of the substrate, and a transparent protection layer 310.
In the conventional photoelectric conversion apparatus, the thicknesses of the first insulating layer, the a-Si:H layer, and the n+-type a-Si:H doping layer are set to be values to satisfactorily obtain photoelectric conversion characteristics in the photoelectric conversion element unit, switching characteristics in the TFT unit, capacitor characteristics in the accumulation capacitor unit. The thicknesses of these layers are respectively about 0.3 .mu.m, 0.6 .mu.m, and 0.15 .mu.m. The second conductive layer must have a thickness of about 1 .mu.m since a signal from the photoelectric conversion element must be transferred to an individual signal wiring in the matrix signal wiring unit through the three layers having the film thicknesses described above.
Therefore, the second insulating layer must have a thickness of about 2 to 3 .mu.m to cover and flatten steps of the photoelectric conversion element unit, the TFT unit, and the matrix signal wiring unit.
However, in the conventional photoelectric conversion apparatus with the matrix wiring unit, the following technical subjects are left unsolved.
More specifically, when the second insulating layer comprises an inorganic insulating film of SiN, mirocracks are formed on the photoelectric conversion element unit, the TFT unit, and especially, in the stepped portions of the matrix wiring unit. As the film thickness is increased, internal stress in the film is increased, and the film may be peeled.
When the second insulating layer comprises an organic insulating film of polyimide, it can be formed with good step coverage without causing microcracks. However, it is difficult to form contact holes.
Methods of forming contact holes can be largely classified into wet etching and dry etching.